It is known to use two or more coaxial dielectric ceramic resonators coupled together to create a filter for use in mobile and portable radio transmitting and receiving devices, such as microwave communication devices. Likewise, two or more re-entrant dielectric ceramic resonators can be coupled together to form such a filter. Resonators in a filter are designed to resonate just one frequency and this frequency is known as the resonate frequency of the resonator. FIG. 1 shows an example of a three-pole filter using three quarter-wavelength coaxial dielectric ceramic resonators coupled together. The coupling method shown in FIG. 1 is a known technique of coupling resonators by providing an aperture or IRIS between the resonators. IRIS is a passage between resonators that allows electrical and magnetic fields of the resonate frequency to pass from one resonator to another. The filter includes an input and an output. The input is usually radio frequencies signals from an antenna or signal generator. The filter only allows the resonate frequency of the resonators and its harmonics to pass through the filter and on to the output. The number of resonators used determines the characteristics of the passing signal, such as bandwidth, insertion loss, skirt response and spurious frequency response. The disadvantage to such filters is that the resonators not only allow the first harmonic of design frequency to pass, but also allow the other associated higher order harmonics of that frequency to pass through the filter. These higher order harmonics are known to interfere with other electronic devices.
It is an object of the present invention to a filter to prevent the passage of higher order harmonics of a design frequency.
It is an object of the present invention to provide a method of coupling resonators.
The present invention is a filter and a method of making a filter to remove unwanted frequency harmonics associated with current filters. The filter is made up of resonators, such that the filter resonates a design frequency. Whereby, at least two resonators are coupled together between an input and an output and at least one of the resonators is of a different design from other resonators, such that the resonator of a different design resonates the same design frequency as the other resonators and resonates different higher order harmonic frequencies than the other resonators. The present invention also provides methods of improving skirt response for a filter, as well as other response properties of the filter. One way to improve the filter""s properties is where at least one of the resonators in a filter is reversed in orientation as compared to the other resonators. Another way is where at least one of the resonators is reversed in orientation electronically by employing electrode coupling on a top and bottom surface of the filter.